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Project Progress

Our initial premise for using Fe isotopes as a biosignature was that the relative mass differences of the four Fe isotopes (54Fe, 56Fe, 57Fe, and 58Fe) would be sufficiently small that inorganic processes would produce minimal fractionations, but that biologic (metabolic) processes would still produce measurable fractionation that would be outside the “noise level” of abiologic effects. In terms of nomenclature used to describe Fe isotope variations, we define: (SEE FORMULA IN ELECTRONIC DOCUMENT) where the reference value is the 56Fe/54Fe ratio of the bulk Earth and Moon (Beard and Johnson, 1999).

Our results obtained in the initial phase of our NAI-funded research has clearly demonstrated the great potential that Fe isotopes have for a robust biosignature. Much of our work in the first year of NAI-funding focussed on developing the analytical methods required for high-precision Fe isotope analysis (Johnson and Beard, 1999). Our work to date has shown:

* There is a constant and apparently homogenous Fe isotope “baseline composition” for igneous rocks from the Earth and Moon (Figure 1; Beard and Johnson, 1999).

* Fe-reducing bacteria produce a sustainable isotopic fractionation, where the 56Fe/54Fe ratio is <sub>1.3 lighter than that of ferric oxide substrate, indicating a clear biological fractionation (Figure 1; Beard et al., 1999).

* Fe isotope compositions of sedimentary rocks that may have had a biologic component to their genesis record Fe isotope variations on the order of 1-2 â?°, similar to that observed in biologic experiments (Figure 1; Beard and Johnson, 1999; Beard et al., 1999).

(Figure 1 — Summary of Fe isotope studies — can be found in the electronic document).

In addition, we have started looking at modern microbial systems (groundwater studies) to evaluate the timescales and preservation potential of Fe isotope biosignatures. Fe(II) groundwater plumes have consistently low d56Fe values over periods of one year, and biogenically-deposited Fe-oxides have similarly low d56Fe values (Figure 1). In parallel with these NAI-funded projects, we have started a detailed study of Banded Iron Formations (funded by NSF), where large variations in d56Fe values have been found that may be related to biologic activity. We have also looked at Fe-Mn nodules (not to be confused with the crusts analyzed by Zhu et al., 2000), which may form by mobilization of sediment iron by Fe-reducing bacteria; the isotopic compositions of large nodules are strikingly similar to those produced by Fe-reducing bacteria in the lab (Figure 1. — Summary of Fe isotope studies conducted at U.W. Madison Beard & Johnson, 1999; Beard et al., 1999; Johnson and Beard, 1999; Beard, Johnson, Skulan, Nealson, Cox, Sun, & Gerdinech, unpublished data — can be found in the electronic document).

An additional effort in the first two years of NAI work has been construction of a new laboratory and accompanying instrumentation that will enable us to pursue Fe isotopes at a much faster rate and at significantly greater precision than any other laboratory in the world. In 1999 we completed the funding package required for purchase of a next-generation Multi-Collector, Inductively-Coupled Plasma Mass Spectrometer (MC-ICP-MS) that is ideally suited for ultra-high-precision analysis of Fe isotopes. This $800K instrument is now installed in a new $100K clean lab at U.W. Madison; funding for the $900K project came from U.W. Madison ($400K), the NASA Astrobiology Institute ($250K) and the National Science Foundation ($250K). The new MC-ICP-MS we have now installed at U.W. Madison is the only instrument that is capable of removing common molecular isobars on the Fe mass spectrum, including 40Ar14N on 54Fe, 40Ar16O on 56Fe, and 40Ar16OH on 57Fe. Past work using first-generation MC-ICP-MS (e.g., Zhu et al 2000; Anbar et al., 2000) attempted to minimize interferences by analyzing very large samples (up to 20 ppm), and not all Fe isotopes were measured. Our new NAI-funded MC-ICP-MS instrument removes the Argide isobars that will allow us to make ultra-precise Fe isotope measurements (a factor of five higher precision than any other lab) on all of the Fe isotopes, with a minimum of sample consumption (50-100 ppb solutions; up to three orders of magnitude lower than any other lab).

Finally, we have begun an exhaustive set of inorganic and abiologic experiments to determine the nature and extent of abiological processes that can fractionate Fe isotopes, and to asses the possibility that these effects could be preserved in the rock record. Such work is clearly required to establish Fe isotopes as a unique biosignature. These experiments also serve as abiological controls for our biological experiments. This work is also important in light of three recent studies that have either criticized our proposal that Fe isotopes may be a robust biosignature (Zhu et al., 2000; Anbar et al., 2000), or proposed significant inorganic isotopic fractionation (Polyakov and Mineev, 2000), which, if true, would significantly reduce the power of Fe isotopes as a biosignature.

Much of the “debate” on applying Fe isotope variations as a biosignature stems from confusion over kinetic versus equilibrium issues. Biologic or “vital” effects might be best thought of as “sustained kinetic fractionations” (where metabolism or other biologic effects supply the driving force that keeps the system from obtaining thermodynamic equilibrium); we interpret our data for Shewanella Algae as a metabolically-sustained kinetic isotope fractionation, which we have measured in runs up to 23 days in length (Figure 1). Longer-term (</sub> one year) isotopic anomalies have been measured for Fe(II) groundwater plumes that are sustained by Fe-reducing bacteria (Figure 1). Such long-term kinetic fractionations are likely to be preserved in nature, which, of course, is an essential component for a biosignature.

Recent criticism of Fe isotopes as a biosignature has used apparent Fe isotope fractionations of several per mil on ion-exchange columns (used to chemically separate Fe from complex natural samples) to argue that similar ranges in Fe isotope compositions may occur abiologically in nature (Anbar et al., 2000). Unfortunately, Anbar et al. (2000) did not realize that they did not attain isotopic equilibrium in their system, which has been known to be a major issue in ion-exchange columns since the 1930’s. Our work has shown that attainment of isotopic equilibrium is never achieved at the high flow rates used by Anbar et al. (2000); the lack of isotopic equilibrium produces strong vertical isotopic gradients in the columns that do not come to isotopic equilibrium even after elution of 99% of the Fe. We contend that such extreme and short-term kinetic isotope fractionations are unlikely to be seen in nature, and would not therefore give a “false positive” biosignature.

We summarize the results of our inorganic experiments to date in Figure 1, which is organized by time scale, emphasizing the need to separate equilibrium, long-term kinetic, and short-term kinetic fractionations in regard to application of Fe isotopes as a biosignature in nature. Our working hypothesis is that while short-term kinetic Fe isotope fractionations are interesting, they have little applicability to nature. Of more importance to using Fe isotopes as a biosignature are the long-term, biologically-sustained Fe isotope anomalies we have measured in the lab and in natural groundwater systems, and equilibrium Fe isotope fractionations in inorganic systems; note that although we find a short-term (hours) kinetic Fe isotope fractionation during acid-hydolysis of hematite, under equilibrium timescales (months), there is no measurable equilibrium isotope fractionation. These results counter recent proposals that abiologic Fe isotope fractionation is important in nature, and confirm our initial premise that Fe isotopes may uniquely identify life.

For the future, we will continue with our biological experiments with the JPL group, including new experiments on Fe-oxidizing bacteria. In addition, we have a comprehensive plan for additional abiologic and inorganic experiments.

(Figure 2. — Summary of kinetic and equilibrium Fe isotope fractionations for inorganic systems, as well as microbial systems in the lab and nature. Note that the effect of kinetic isotope fractionations on the use of Fe isotopes as a biosignature depends upon the timescales involved — can be found in the electronic document).